Control system



C. E. DEAN CONTROL SYSTEM Filed Jan. 28, 1931 4 Sheets-Sheet l INVENTOR(ar/as eafz BY @m1, 'TEM/6g, M mi W,

ATTORNEYS C. E. DEAN CONTROL SYSTEM May 1l, 1937.

4 Sheets-Sheet 3 Filed Jan. 28, 1931 nsw INVENTOR dr/es f ea/y May l1,1937. C. E DEAN 2,080,115

` CONTROL SYSTEM Filed Jan. 28, 1951 4 Sheets-Sheet 4 INVENTOR (Zar/effe/i @ma/MSM ATTORNEYS Patented May 11, 1937 UNITED STATES CONTROLSYSTEM Charles E. Dean, Bayside, N. Y., assignor to HazeltineCorporation, a corporation of Delaware Application January 28, 1931,Serial No. 511,715

22 Claims.

This invention relates to electrical communication or signal systems forthe transmission of high or carrier frequencies, modulated by a band ofaudio or other signal frequencies, and more particularly to such systemsas are tunable over a range of carrier frequencies. The inventionincludes means to automatically attenuate the higher audio or signalfrequencies to offset the varying loss of components at the edges of thecarrier frequency channel and thus obtain a substantially uniformoverall audio-frequency response throughout said range.

The paramount object of this invention is to improve the operatingcharacteristics of communication systems without increasing the amountof manipulation and adjustment necessary on the part of the user oroperator.

A specific object of this invention is to provide an arrangement toalter the gain-frequency characteristic of the low-frequency orsignal-amplifier portion of a radio receiver in accordance with thetuning of the radio-frequency portion of the receiver, whereby thetendency for attenuating the higher-frequency elements of the reproducedsignal, which accompanies reception on the lower radio frequencies ofthe response range, is offsetA and the fidelity of signal reproductionis enhanced. There is thus provided in the case of broadcast receivers aspecies of tone control in which the gain-frequency responsecharacteristic of the audio circuit is automatically altered to givefaithful reception of broadcasts over the entire tuning range.

Well-designed receivers heretofore used for the reception ofradio-telephone broadcasting have been characterized by sharperselectivity curves at the lower radio frequencies, resulting indiminution of the high pitches in the reproduced sound in comparisonwith reception at the higher radio frequencies. On account of thefact-.that

the effect of over-emphasized high pitches is very disagreeable, thebest design has been to provide for approximately accurate reproductionof these pitches at the upper frequency end of the broadcast range,leaving the reception deficient in this regard at the medium andespecially the lower broadcast frequencies. This condition has been themore unfortunate because the better broadcasting stations generallytransmit upon the low and medium frequencies in the broadcast range.

This condition is remedied by the application of the present inventionwhich makes three provisions for increasing the tone fidelitycharacteristic of the receiver. The first is the provision of aspecially designed audio-amplifier circuit, which (Cl. Z50-20) iselectrically linked to the carrier frequency antenna circuit throughmeans including a frequency changer or modulator, and which, except forthe effect of the tone control, has an increased transmission efficiencycharacteristic for 5 the high signal frequencies. The second is theprovision of a tone control including a variable impedance forcontrolling the frequency response characteristic of the audio circuit.The third is the provision of a mechanical linkage between the tonecontrol and the carrier-frequency tuning mechanism such that the ratioof high-tolow-frequency response of the audio circuit is automaticallyincreased at a predetermined rate as the tuning circuit or circuits forthe choice of stations is tuned to the lower radio-frequency carriertransmissions. In this way, a high and substantially uniform tonefidelity characteristic is obtainable over all of the plurality ofradiofrequency channels constituting the broadcast range or frequencyspectrum. Furthermore, this association of the two controls is done insuch a manner that operation of the resulting single controlsimultaneously and automatically accomplishes both electrical effects.

The invention will be understood more fully and other objects made clearfrom the following description read in connection with the drawings, andfrom the appended claims.

Referring to the drawings, Fig. l represents the general principle` ofthe invention;

Fig. 2 gives selectivity curves of a typical present-day broadcastreceiver;

Fig. 3 shows the changing discrimination of such a receiver against sidefrequencies conveying pitches of 2000 and 4000 cycles per secondreceived from radio stations transmitting upon various carrierfrequencies;

Fig. 4 gives various fidelity curves showing the improvement effected bythe invention; Y

Fig. 5 is a circuit diagram of a radio receiver embodying the presentinvention;

Fig. 6 is a view of the tuning condenser unit of the receiver of Fig. 5showing the added control of the audio gain-frequency characteristicmounted on the tuning shaft; q

Fig. 7 is a schematic diagram of the application of the invention to aradio transmitter.

In Fig. l are shown a pair of input terminals 21, a transmission circuitA, operating at frequencies received at the input terminals, afrequencychanging device B, a transmission circuit C, operating at someor all of the frequenciesproduced by the frequency-changing device B, apair of output terminals 28, a control wheel 29, and shaft 55 3l, andtwo shafts, 30 and 30 operated simultaneously by rotation of the wheel.The rotation of the shafts 30 and 30' produces changes in thetransmission characteristics of the two transmission circuits, A and C,respectively, whence rotation of the control wheel changes thetransmissioncharacteristics of these two portions of the systemsimultaneously and accomplishes one of the objects of the invention.

The transmission characteristics Vcaused to vary by motion of shafts 30and 30 and the direction and amount of variation are chosen accordingLto the requirements of the particular system. It has been known thatthe gain in circuit C mightbe advantageously varied in accordance" withVthe gain in circuit A or in accordaneefwithfv'the pesition of afrequency selecting control in circuit A. It should be noted that achang-Ae ingain or in simple gain is a uniform change for allfrequencies, for example, an increase of ten decibels,

and does not introduce any change in the relative transmission Veiciencyfor the'rvarious frequencies.` The'present invention is directed towardthese cases in which a change of frequency characteristic orla'nychan'geother than gain in circuit C is'd'esirable, and toward those'cases inwhich any change inthe transmission characteristicof circuit'Cisdesirable accompanying any change in circuit A`except variation ingain or frequency selectivity. The term frequency chai'- acteristic orgain-frequency characteristic is used to designate transmissionefficiency as a function offrequen'cy, of Winch frequency selectivityis"avspecial'c'as`e in which a certain band of frequencies i'svreceivedand all others are substantialljyy rejected, the frequency bandwhich isreceived usually being variable and located at any desirableposition ina wider operating range. Variation in frequency selectivity' isgenerally cauedrturiiii'g.

Forexample', in 'the' broadcast receiver described below, a change'inIfrequencyv characteristic in ciieuit' C accompanying a change infrequency selectivity in circuit A is highly desirable for the reasonthat a high'degreeof fidelity of reproduction may thus be' obtainedthroughout the radio-frequency range over which the setv can'be tuned. AA

' I'n'ot'her cases, such as picture transmission and television, thepreservation of' phase relations between vthe signal coInpOIlents isimportant Vand the invention can be used to effect desired automaticphase adjustments inone'; portion ofthe system accompanying Achange oftransmission character'- istics another portion' which is separated by afrequency-changingdevice. In still other cases it maybe desired'tovarythe power handling capacity o'f one portion of a system in'accordancewith a transmission adjustment of another portion operating at adifferent frequency. Furthermore,`any combination of changes intransmission characteristics may( beobtained in one such portion,vacoompafnying o any combination of changes in the other portion.

o Before describingthe application of the inventiontoa broadcastreceiver, it is desirable to consider vcertain characteristics andlimitations of wellf-'desig'ned'.receivers of l the present time. InFig. 2 arev givenv selectivity curves of such a receiver, plottedA inthe conventional manner. TheseA curves are obtained by connecting therecei'ver to ai standard antenna, feeding in current of a certainfrequency, for example 600 kilocycles per* second,tuningy the receiver,and then noting the relative voltages at other'frequencies necessary toproduce the same output Without changing the tuning of the receiver. Alow modulation frequency, 400 cycles, (the phrase per second is omittedhere and hereafter in accordance with common practice) has beenstandardized for this purpose and permits the regular detector and audioamplifier of the receiver to be used as convenient auxiliaries to theoutput measuring equipment. The scale of ordinates used in plotting theresults is logarithmic since this gives a much closer approximation tothe intensity characteristics of the human ear than does a linear scale.The curves show the discrimination in the tuned circuits `of thereceiver against various frequencies when the set is tuned t0 theindicated frequency. For example',l when the set is tuned to 1000kilocycles it is seen that a frequency of 1020 kilocycles must have itsvoltage multiplied by 1'75 in order to produce the same effect as itwould if its frequeney'were 1000 kilocycles. rIhat is, an unwantedsignal of 1020 kilocycles is reduced by a voltage ratio of one part in175 if the set is tuned to 1000 kilocycles. In'this way the signals fromundesired'stations are generally suppressed leaving only the desiredspeech or music. The discrimination against unwanted signals a givennumber of kilocycles away from the frequency of tuning is seen to begreater when the set is tuned to 600 kilocycles than when tuned to 1000kilocycles, and greater when tuned to 1000 kilocycles than when tuned to1400 kllocycles. This illustrates the well-known fact that theselectivity is greater when tuned to the lower broadcast frequencies.

It is well known in the radio art that a highfrequency current, whoseenvelope varies in accordance with the amplitude of a signal wave in themanner employed in a broadcast transmitter, is found upon mathematicalor electrical analysis to consist of currents of the carrier frequencyand of other frequencies located below and above the carrier frequencyby intervals equal to the frequency components of the signal wave. Thus,a 1000-cycle tone with the second harmonic of 2000 cycles would betransmitted by a station on 1400 kilocycles with the simultaneousradiation of the transmission band comprising 1398, 1399, 1400, 1401 and1402 kilocycles. As stated above, the l400-kilocycle frequency is calledthe .carrier frequency. The other frequencies are called sidefrequencies, and the group of side frequencies on each side of thecarrier is called a sideband. (The term modulation frequencies issometimes used to designate side frequencies but also is frequently usedto designate frequency components ofthe modulating signal.) Themodulated wave may therefore be said to consist of a lower sideband, anunmodulated carrier component, and an upper sideband. In the cas'e justconsidered where a 1000-cycle tone with a 200G-cycle harmonic istransmitted on a 1400- kilocycle carrieigthe 1398 and 1399-kilocyclefrequencies are the lower sideband and the 1401 and 1402-kilocyclefrequencies are the upper sideband. In accordance with the generalstatement at the beginning of this paragraph, the 1399 and1401-kilocycle components (in cooperation with the carrier of 1400kilocycles) convey the fundamental of 1000m/cles, the essential pointbeing that 1399 is 1 kilocycle or 1000 cycles below the carrier and 1401is an equal interval above. Similarly, the frequencies of 1398 and 1402kilocycles convey the second harmonic of 2000 cycles. Higher-frequencycomponents of a signal are similarly conveyed in the transmittedhigh-frethe dotted line.

quency wave by frequencies in the two sidebands lying farther from thecarrier frequency.

In the case of speech and music the low pitches are conveyed in thetransmitted wave by side frequencies near the carrier frequency,intermediate pitches are conveyed by side frequencies correspondinglymore removed from the carrier, and the highest pitches are conveyed bythe farthest removed side frequencies Pitches between 3000 and 5000cycles are conveyed by part of the lower sideband extending from a point5 kilocycles below the carrier to a point 3 kilocycles below thecarrier, and by part of the upper sideband occupying a symmetricalposition extending from 3 to 5 kilocycles above the carrier.

Referring again to Fig. 2 it can be seen that the discrimination againstother signals, (suchas from other stations 10, 20, 30 kilocycles or moreaway in frequency) extends rto some extent to frequencies within vekilocycles of the frequency of tuning. This condition results inattenuation of the farthest removed side frequencies of the desiredsignal, commonly called sideband cutting designating that the twosidebands are cut or reduced in amplitude at their edges, the lowersideband at its lower edge and the upper sideband at its upper edge.

In case the audio amplifier and speakerhave approximately equalcharacteristics for the various audio frequencies, sideband cuttingproduces a diminution of the high pitches in the reproduced sound. Thisloss causes a reduction .in the normal brilliance of music and crispnessand clarity of speech, and thus mars the quality of the reproducedsound. To avoid this defect without altering the audio amplifier wouldrequire that all the selectivity curves of Fig. 2 have, between thelimits -5 to -1-5 kilocycles, the rectangular band-pass characteristicindicated by Efforts toward the development of circuits with suchcharacteristics have been made, but the results have been only partiallysuccessful and have not found wide acceptance.

At any one carrier frequency the loss of high pitches in a typical radioreceiver may be compensated by suitably increasing the gain of the audioamplifier for the higher pitches `without material change of the lowpitch gain. However, as seen in Fig. 2 the selectivity changes with thelocation of the transmission band in the frequency spectrum, so thatcorrection of the sideband cutting at one frequency still leaves itincorrect at all other frequencies. In Fig. 3 are shown the effects ofsideband cutting based upon the curves of Fig. 2 near frequency oftuning. The upper curve in Fig. 3 gives the transmission of sidefrequencies conveying a 200G-cycle pitch through the tuned circuits ofthe receiver in terms of the transmission of the carrier frequency whichis naturally taken as a standard. A logarithmic scale of ordinates isused here for the same reason as in Fig. 2. The upper curve here shows,for example, at 600 kilocycles that a pitch of 2000 cycles is reduced to60% due to sideband cutting, and at 1000 kilocycles is reduced to 82%.The lowercurve shows the much greater attenuation suffered in thereception of a pitch of 4000 cycles, and it may be seen that this isespecially severe at the low-frequency end of the broadcast band. Theserious character of the situation may be realized when it is noted thatthese curves are representative of good design of the present day. Infact this lack of fidelity'in the reproduction of high pitches fromstations on the lower broadcast frequencies is the main fidelity defectYof the typical-goo receiver of current manufacture. The defect is themore unfortunate since, as a result of various causes including thebetter propagation of radio waves of the lower frequencies in thebroadcast band, the larger and more important broadcasting stations aregenerally found assigned to these frequencies. i

In Fig. 4, curves A and B represent the overal1 fidelity of response, at600 and 1400 kilocycles, respectively, of a representative present-daybroadcast receiver, and show, as an example, the deficienciesencountered in tonal reproduction when receiving signals of the lowerand intermediate broadcast frequencies. These curves include the effectof the characteristics of all components of the radio receiver includingthe speaker. i

The ordinates for all the curves in Fig. 4 are obtained by applying aconstant input voltage, noting the amount of output acoustic energy forthe particular audio frequency, taking the square root thereof, andcomparing this gure with the corresponding result for 400 cycles whichis taken as 100%. Logarithmic graduations are used for both coordinatescales to approximate ear characteristics, and in the case of thefrequency scale, also to prevent crowding of the lower frequencies. InFig. 4, curve C gives the characteristics of the audito system only,without the invention.

The various curves coincide over the low-frequency range on account ofthe fact that the radio-frequency amplifier introduces no losses for lowpitches, wherefore the audio-amplifier characteristic determines theoverall character- 'i istic for all frequencies of tuning. Curve D givesthe characteristic of the audio system, with the invention, when thereceiver is tuned to 600kilocycles. Curves E and F give thecharacteristic of the audio system when the receiver` is tuned to twohigher frequencies in the broadcast band. As the tuning is changed from600 kilocycles to i400 kilocycles the characteristic of the audio systemvaries gradually from curve D, through curves E and F' to curve` C. By asuitable design of the means for varying the characteristic of audioamplier, the overall fidelity of the receiver may be made the same forall. frequencies of tuning, and it may be made to have a. curve such asB in the figure. In this way, as good fidelity is obtained over theentire broadcast range as vwas formerly had only at the higher frequencyportion of the tuning range. Curve B represents a practical compromisebetween good high-pitch reproduction and desired attenuation of staticand components of unwanted signals. For example, thev upper sidefrequencies of the station on the next channel 10 kilocycles below, andthe lower side frequencies of the station on the next channel 10kilocycles above would produce some very high-pitched interference ifthere were not appreciable,attenuation immediately above 5000 cycles.

Fig. 5 shows the circuit of a radio receiver of present-day designembodying thisinvention. A radio-frequency amplifier providing threestages of radio-frequency amplification in cascade, detector, ordemodulator, and an audio-frequency amplifier providing two stages ofaudio-frequency amplification comprise the receiving system. Theselective circuits of the high frequency amplifier may be tuned by anyselective circuit elements as, for instance, variable eondensers I, 2, 3and d. It is to be understood that the application of the invention isnot confined to receivers of the type shown', but can be used with allsystems well known in the art, where there is a radio-frequency tuningcircuit for the choice of stations, a detector, rectifier, modulator, orother frequency changer, and an audio circuit conveying the detectedsignal. For the purpose of clarity the power supply unit is not shownhere, as its function in relation to the receiving system is well-knownin the art, and whether batteries or alternating current rectifyingarrangements are usedfor the supply, it has no direct bearing on thisinvention. The volume control 6, increases the volume, as lt is rotatedto the right, due to a double action of increasing the shuntingresistance 8, across the primary I3, of the antenna transformer I4, andalso reducing the grid bias on the three radio-frequency amplifyingtubes I5, I6 and I'I, by reducing the bias resistance 9, includedbetween the cathodes I0, II and I2 of these tubes and ground. Thisdouble volume control furnishes' the necessary attenuation forsatisfactory use of the set with very strong signals such as frompowerful local stations, and also allows the employment of the highavailable sensitivity when desired. For the sake of simplifying thedrawing the cathode heater and filament supply leads are omitted. Theseare connected to suitable windings on the power transformer inaccordance with well-known practice. A tone control comprising aresistance and a capacity 'I in series is electrically connected inshunt with the secondary of the transformer I9. The total impedance ofthe shunt path thus provided may be made adjustable by making eitherresistor 5 or capacityr'I variable.

As indicated by the heavy dotted line, the four variable tuningcondensers I, 2, 3 and 4, and the variable resistor 5, of the shunt pathacross the secondary I8 of the push-pull input transformer I9 'areoperatively connected and are therefore controlled simultaneously. Asthe capacitance of the condensers I, 2, 3 and 4 is increased to tune theselective circuits from the high to the low frequency end of the tuningrange, and stations on successively lower carrier frequencies are tunedin, the value of resistor 5 is increased, thus reducing the loss of thehigher pitch audio currents through condenser 'I in series with theresistance 5 and gradually increasing the high-pitch audio-frequencygain.b Without the loss for high pitches the audio amplifier has greatergain for high pitches than for low so that a material degree ofcompensation for the effects of sideband cutting is obtained. Thisgreater audio gain for high pitches when receiving the lower broadcastfrequencies is obtained by the lesser eifect of resistance 5 andcondenser 'I and also by the greater eflici'ency for high pitches oftransformer I9. This transformer Vczharacteristic results from theappreciable leakage reactance of the transformer, sufficient to resonatewith the distributed capacitance of the windings and the circuitcapacitance at a frequency near the upper frequency of the band which itis desired to reproduce. By the use of low distributed capacitance andsufficient leakage reactance the voltage ratio of the transformer can bemade 2 to 4 times as great at the resonant Yfrequency as at lo'w audiofrequencies. In this way the invention may be employed to improvesubstantially the action of the receiver, although in the simpleforrndepicted in Fig. 5 it may not accomplish the near-ideal compensationdescribed in connection with Fig. 4. With selectivit'y curvesapproximately 'as shown in Fig. 2, and

under conditions such as usually found in present-day receivers theorder of magnitude of the maximum value of resistor 5 may be 1 megohmand the order of magnitude of condenser 'l may be 0.003 microfarad. Thedetermination of such values for any conditions by either computation orexperiment is well known in the art and presents no diiiiculty.

Fig. 6 is a perspective drawing of the variable condenser unit of thereceiver, the circuit of which has been indicated in Fig. 5, showingthevariable resistance unit 5 mounted on the same shaft 2U with thecondensers I, 2', 3 and 4. This drawing serves to illustrate a simplemechanical assembly by means of which the Variable resistor element canbe attached to the tuning shaft in order to be actuated conjointlytherewith and bring about the variation in the audio gain-characteristicin accordance with the position of the tuning control. Other equivalentdevices may, of course. be employed.

It will be clear to any one skilled in the art of electricalcommunication that this invention may be applied to any desired extentin a radio receiver. For example, series selective elements may beemployed in the audio amplifier and varied automatically by motion ofthe tuning element as already described. Additional variable shuntselective elements may also be utilized.

By the use of a suitable arrangement of series and shunt variableselective elements practically perfect fidelity to 5000 cycles can beobtained for all broadcast radio frequencies. A low-pass lter, with asharp cut-off at 5000 cycles can be included, and thus the idealcombination of maximum fidelity and freedom from interference for thepresent 10-kilocycle separation of broadcasting stations can beobtained.

If it is desired to use the audio amplifier for the electricalreproduction of phonograph records, this may be done without seriousloss of fidelity by arranging the phonograph switching control so as tobe actuated by the tuning control immediately after the latter is movedvbeyond 1500 kilocycles. This switching arrangement, from radioreproduction to phonograph records, has been used in some types ofreceivers sold in the United States. With the present invention theaudio system has nearly equal transmission characteristics for thevarious pitches when the tuning is at 1500 lrilocycles. The onlydeparture in a thorough application of the invention is a slightemphasis on the high pitches to offset the small amount of sidebandcutting at 1500 kilocycles. If desired this may be corrected and anyselective characteristics of the electric pick-up reproducer alsocorrected by the provision of a suitable selective circuit between thepick-up and the point where connection into the transmission circuit ofthe receiver is made.

It should be noted that the employment of this invention in a radioreceiver does not preclude the use of a manual audio gain-frequencycontrol, such as is generally called a tone control.

`That is, the use of the automatic tone control of 'this invention doesnot necessarily replace the usual tone control. Manual tone controlshave already been used in radio receivers, and have certain advantages.By the use of them persons of defective hearing, who usually havegreater loss for a certain pitch range, may strengthen this part in thereproduced sound. It may also be used to make changes to correct for theacoustics of the room where the receiver is used. An-

other use for a manual gain-frequency control is to reduce the highpitches when loud static is present, thus improving the generalreception despite the partial absence of the high pitches in thereproduced sound. If any of the broadcasting stations heard aredeficient in high pitches or low pitches, but not both, the manualcontrol may be used to compensate for this deficiency.

Referring to Fig. 7, a signal amplifier 2| is associated withtransmittingequipment 23, comprising an oscillator and a modulator.Desired carrier frequencies can be selected by rotation of thecarrier-frequency control dial 22 which in turn rotates shaft 24 andthereby actuates gainfrequency control elements 25 and 2S of the signalamplifier 2|. The transmission characteristic of the signal amplifierthereby can be varied in accordance with variations in the transmissioncharacteristics of the carrier-frequency gener'- ator.

rlhe invention claimed is:

l. In an electrical communication system having one portion operating atfrequencies within a certain band, said band being variable in itslocation in the frequency spectrum, and another portion transmitting atleast certain frequencies under all operating conditions, thecombination of a control means to vary the frequency at which oneportion operates, and means interconnected with said control means foraltering the gfainfrequency characteristic of said other portion as thelocation of said band is Varied by said control 2. A signaling systemincluding a radio-frequency amplifier linked with an audio-frequencyampliiier, each of said amplifiers being responsive over a frequencyrange, at least one of said amplifiers being adjustable to amplifyselectively desired frequencies within its range, and means including asingle control member associated with both amplifiers for varying theratio of high-tolow frequency response of one of said amplifiersautomatically as the frequency response of the other of said amplifiersis selec-tively adjusted.

3. In a radio signaling system operative over a plurality ofradio-frequency channels, each of said channels comprising aradio-frequency carrier wave modulated by audio frequencies, aradio-frequency amplifier selectively tunable to said channels by acontrol member, an amplifier for said audio frequencies, and meansassociated with said audio-frequency amplifier and operable by saidcontrol member .for varying the ratio of high-to-low audio-frequencyresponse automatically as said radio-frequency amplifier is tuned 'fromone to another of said channels.

4. In a radio signaling system operative over a range of adjacentradio-frequency channels, said range terminating in a high and a lowend, each of said channels comprising a radio-frequency carrier wavemodulated by a band of audio frequencies, a receiving system selectivelytunable to said channels in which toward the lower end of said range atendency for attenuating the higher audio frequencies is produced,including: a demodulator, an amplifier for said audio-frequencies, meansassociated with said amplifier for adjusting the audio-frequencyresponse of said amplifier to accentuate the higher audio frequenciesand a control member to operate said means simultaneously as saidreceiving system is tuned toward the lower end of said range, wherebythe fidelity of reproduction is maintained substantially uniform.

5. A radio receiving system comprising a plurality of high-frequencyamplifying stages in cascade, tuning means for each of said stages fortuning said system over a range of frequencies, a detector, a pluralityof stages of audio-frequency amplification, means associated with one ofsaid audio-frequency amplifier stages for adjusting the ratio ofhigh-frequency to low-frequency response of said audio-frequencyamplifier stage, and means for simultaneously operating saidlast-mentioned means and said tuning means to adjust the frequencyresponse ratio automatically as said receiving system is tuned over saidrange in frequencies.

6. In a radio receiving system, selective circuits tunable to respond toa range of frequencies, a detector connected thereto and followed by anamplifier for amplifying audio-frequency currents, adjustable meansassociated with said amplifier-for altering the audio-frequency responsethereof, and control means for tuning said circuits and simultaneouslyand gradually increasing the audio-frequency gain for high pitches assaid circuits are tuned toward the lower-frequency end of their responserange.

7. In a radio receiving system, the combination of a plurality of stagesof radio-frequency amplifiers tunable conjointly by means of variablecondensers, a detector connected thereto and followed by anaudio-frequency amplifier, means operatively connected to said variablecondensers and electrically associated with said ,i

audio-frequency amplifier for simultaneously varying the gain-frequencycharacteristic thereof with the tuning of said radio-frequencyarnplifying stages.

B. In a radio receiving system including radiofrequency amplifyingstages tunable over a range of frequencies, a detector and anaudio-frequency amplifier, the combination of means for tuning saidamplifying stages, associated with said audio-frenquency amplifierincluding a shunt path, for controlling the tone thereof, and means forsimultaneously operating said lastmentioned means and the tuning meansof saidv radio-frequency amplifying stages, whereby the ratio ofhigh-to-low frequency response of said audio-frequency amplifier isautomatically controlled as the system is tuned.

9. In a radio receiving system, selective circuit elements tunable torespond to a range of frequencies, a rectifier, an audio-frequencyamplifier, means electrically connected to said ampliiier comprising ashunt path which includes resistance and capacity in series, the totalimpedance of which is adjustable, and a single control means forconjointly and simultaneously adjusting said impedance and varying thetuning of said circuit elements.

10. In a radio signaling system operative over a plurality ofradio-frequency channels, each of said channels comprising aradio-frequency carrier wave modulated by a band of audio frequencies, aradio-frequency amplifier selectively tunable over said range, wherebythe modulation frequencies at the edges of said channel are attenuated,a rectifier, a plurality of stages of audio-frequency amplification,means associated with one of said audio-frequency amplifying stages forcontrolling the audio response thereof, and a single control means forsimultaneously tuning said amplifier and operating said lastmentionedmeans and thereby automatically maintaining the overall response toaudio frequencies substantially uniform as said radiofrequency amplifieris tuned over said range.

11. In a radio receiving system, a radio-fre- 'compensate for theeifects of quency amplifier tunable over a range of frequencies, adetector, an audio-frequency amplier, circuit elements including avariable impedance associated with said audio-frequency amplifier, and acontrol member to vary simultaneously said impedance and the tuning ofsaid radio-frequency amplier, whereby the gainfrequency characteristicof said audio-frequency amplifier is varied in a predetermined mannerwith the tuning of said radio-frequency am- 12. In a radio signalingsystem, a control member, a carrier-frequency generator adjustable bysaid control member over a range of frequencies, a modulator, a signalamplier and means associated therewith and also operable by said controlmember for controlling the frequency response thereof simultaneously andat a predetermined rate as said generator is adjusted from one carrierfrequency to another within said range.

13. In an electric system for the faithful transmission or reception ofsignals over a plurality of. frequency channels, including two partselectrically linked by a frequency changing device, the method ofoperating said system which consists in varying the frequency selectingcharacteristic of one of said parts and simultaneously varying thefidelity characteristic of other of said parts.

. 14. The method of maintaining substantially uniform tone fidelity inradio receivers having radio and audio-frequency amplifiers, whichcomprises simultaneously varying the ratio of high-to-low frequencyresponse of said audiofrequency amplifier and the tuning of saidradiofrequency amplifier.

15. In a radio receiver, a tuning circuit for the choice of stations, adetector, an audio circuit, a tone control in said audio circuit, and acommon control member for said tuning circuit and saidv tone control.

16. In a radio receiver, a stage of tuned radiofrequency amplication,tuning means in said stage, the selectivity of said stage being variablewith tuning, a detector, an audio-frequency amplier, means forcontrolling the tone of. said amplier, and additional means forrendering said tone control means simultaneously operative with saidtuning means in such sense as to Variation in selectivity of saidradio-frequency stage with` tuning.

17. In combination, a radio-frequency amplifier, means adapted to tunesaid amplier over a band of frequencies and exhibiting a greater degreeof selectivity at one end of said band of frequencies than at the other,a detector, and an audio-frequency transmissionV circuit, and means,mechanically linked with said tuning means, automatically increasing thetransmission efficiency of the higher audio frequencies through saidtransmission circuit as the selec-` tivity of said radio-frequencyamplifier is increased by tuning to said one end of said band offrequencies.

18. In a radio receiving system comprising tuning devices and means forintercepting, amplifying and rectifying modulated electricaloscillations of different transmission frequencies, means operativeafter rectification to reduce the transmission of relatively highaudio-frequency oscillations and to produce characteristic outputvoltage curves which are substantially alike for all transmissionfrequencies, and connections between the last-named means and the tuningdevices to enable said last-named means and the tuning devices to beadjusted in unison.

19. In a system comprising means for intercepting, selecting, amplifyingand rectifying modulated electrical oscillations of different'transmission frequencies, means including a variable shunt resistanceoperative after rectification to produce output voltage curves which aresubstantially alike for all transmission frequencies, and connectionsbetween the last-named means and the selecting means to enable thelast-named means and the selecting means to be adjusted in unison.

20. In a system comprising means for intercepting, selecting, amplifyingand rectifying modulated electrical oscillations of differenttransmission frequencies, means after rectification, comprising acondenser and a variable resistance in series therewith and forming ashunt circuit, operatively associated with the selecting means to beadjusted in unison with the latter to produce output voltage curveswhich are substantially alike for all transmission frequencies.

21. In a radio receiver, a stage of tuned radiofrequency amplification,tuning means in said stage, a detector, an audio-frequency amplier,means for controlling the tone of said amplifier, and additional meansfor rendering said tone control means simultaneously operative with saidtuning means, said tone control means transmitting the higher audiofrequencies most efciently when the tuning means is adjusted to the lowfrequency end of the receiver tuning range.

22. In a system comprising means for intercepting, selecting, amplifyingand rectifying modulated electrical oscillations of dierent transmissionfrequencies, an amplifier for the rectified output, and means,comprising a condenser and variable resistance in series, in shunt withthe input of said amplifier' to eifect characteristic voltage curveswhich are substantially alike for all transmission frequencies, saidvariable resistance being operatively associated with the selectingmeans to be adjustable in unison therewith.

CHARLES E. DEAN.

